Most Linux users are familiar with KDE and GNOME, as both are desktop environments used by most major Linux distributions but there are some really good alternate window manager out there. If you have some old PC with minimum hardware and want to run GUI on it then in this case the list of window manager provided below will definitely help you to chose one ..

XFCE
It is a lightweight desktop environment for Unix operating systems. The main goal is to be fast and light, while being visually attractive and easy to use. It is based on the GTK + toolkit same as GNOME, so if you are Gnome fan, you'll find XFCE very usefull.

Enlightement
Enlightenment is a themeable, fast, flexible, and powerful window manager that is designed to be as configurable as possible in both look and feel. In addition, Enlightenment provides an applet API that provides functionality somewhere between the GNOME applet system and Dock Apps from Afterstep/WindowMaker. The current design aim is for Enlightenment to become a desktop shell..

FVWM-Crystal
FVWM-Crystal aims to create an easy to use, eye-candy but also powerful desktop environment for Linux or other Unix-like operating systems. It uses following programs: FVWM as a window manager and "main core", ROX-Filer as file manager (manages icons on the desktop), xterm, aterm, mrxvt or urxvt as terminal emulators, MPD or XMMS as music players (there's built-in support for controlling these programs), and several other tools for different functions, like setting a wallpaper or making screen shots.

LXDE
"Lightweight X11 Desktop Environment", is an extremely faster, performing and energy saving Linux desktop environment maintained by an international community of developers. It comes with a beautiful interface, multi-language support, standard keyboard short cuts and additional features like tabbed file browsing. LXDE uses less CPU and less RAM. It is especially designed for computers with low hardware specifications like netbooks, mobile internet devices (MIDs) or older computers. LXDE can be installed with distributions like Ubuntu or Debian. Applications running on these systems will run with LXDE. The source code of LXDE is licensed partly under the terms of the General Public License and partly under the LGPL.

IceWM
IceWM is simple and highly configurable. All settings are stored as plain text files on the user's home directory, so it is easy to customize and configure it. You can apply your favorite theme and make it appear Windows Vista, Mac OS X or whatever you want.

Fluxbox
Fluxbox is a windowmanager for X that was based on the Blackbox 0.61.1 code. It is very light on resources and easy to handle but yet full of features to make an easy, and extremely fast, desktop experience. It is built using C++ and licensed under the MIT-License.

Window Maker
Window Maker is an X11 window manager originally designed to provide integration support for the GNUstep Desktop Environment. In every way possible, it reproduces the elegant look and feel of the NEXTSTEP[tm] user interface. It is fast, feature rich, easy to configure, and easy to use.

Window Maker includes compatibility options which allow it to work with other popular desktop environments, namely GNOME and KDE, and comes with a powerful GUI configuration editor, called WPrefs, which removes the need to edit text-based config files by hand.

AfterStep
AfterStep is a window manager for the Unix X Window System. Originally based on the look and feel of the NeXTStep interface, it provides end users with a consistent, clean, and elegant desktop. The goal of AfterStep development is to provide for flexibility of desktop configuration, improving aestetics, and efficient use of system resources.

Some of the distinguishing features of AfterStep compared to other window managers are its low usage of resources, stability and configurability.